Revision as of 15:31, 18 October 2007

This page has links to information about the (relatively) new timer systems for the Linux kernel.
The current linux kernel received a major enhancement to it's timer system (as of about 2.6.21),
which solved a number of problems.

Material needs rework

A bunch of material below this line needs to be reworked to take into account the new
ktimer system by Thomas Gleixner.

Timer Wheel, Jiffies and HZ (or, the way it was)

The original kernel timer system (called the "timer wheel) was based on
incrementing a kernel-internal value (jiffies) every timer
interrupt. The timer interrupt becomes the default scheduling
quamtum, and all other timers are based on jiffies. The
timer interrupt rate (and jiffy increment rate) is defined
by a compile-time constant called HZ. Different platforms
use different values for HZ. Historically, the kernel used
100 as the value for HZ, yielding a jiffy interval of 10 ms.
With 2.4, the HZ value for i386 was changed to 1000, yeilding
a jiffy interval of 1 ms. Recently (2.6.13) the kernel
changed HZ for i386 to 250. (1000 was deemed too high).

Ingo Molnar's explanation of timer wheel performance

Ingo Molnar did an in-depth explanation about the performance of
the current "timer wheel" implementation of timers. This was
part of a series of messages trying to justify the addition
of ktimers (which have different characteristics).

High Resolution Timers

Old timer wheel/jiffy replacement proposals

Jun Sun's "tock" proposal

This systems replaces jiffies and xtime with tocks (arch-dependent), mtime (monotonic time) and wtime (wall time),
and proposes a strategy for migrating to that.

John Stultz

In 2005, John Stultz proposed changes to the timers to use a 64-bit nanosecond value as the base.
He did a presentation and BOF at OLS 2005. (It should be available online)

Timer Tick Thread - LKML July 2005

There was a very long thread about timers, jiffies, and related subjects
in July of 2005 on the kernel mailing list.

The title was: "Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt"

Linus said jiffies is not going away

- still need 32-bit counter, shouldn't be real-time value (too much overhead to calculate)
- high-res timers shouldn't be sub-HZ, but instead, HZ should be high and timer tick should not be 1:1 with HZ
- in other words, have HZ be high (like 2K), have the timer interrupt fire off at some lower frequency,
and increment jiffies by more than one on each interrupt.
- rationale for this is to keep a single sub-system

Arjan had good points about coalescing low-res timers

- 3 use cases:
- low res timeouts
- high res timer for periodic absolute wakeup (wake up every 10 ms, whether last one was late or nt
- high res timer for periodic relative wakeup (wake up 10 ms from now)